1. Field of the Invention
The present invention relates to a vibration power generator that converts vibration energy into electric power.
2. Description of Related Art
In recent years, attention has been paid to energy harvesting, which is to extract electric power from energy widely present in the environment for feeding it to low-power electronic devices, including solar power generation, thermoelectric generation, electromagnetic induction using magnets and coils, and the like. One of such techniques known in the art is an electrostatic induction vibration power generator, which extracts electric power from vibration energy of human bodies, vehicles, machines and the like. Electrostatic induction vibration power generators include a semi-permanently charged film called electret that is placed either on an electrode of a vibrating body or on a fixed electrode opposed to the vibrating body in a device. By changing the electrostatic capacity between the two electrodes, an electric current is induced, and the induced current generates a voltage. An electric power is thus extracted.
FIGS. 9(a) and 9(b) show a conventional vibration power generator. FIG. 9(a) is a cross-sectional view of the vibration power generator when its vibrating body is at a resting position. FIG. 9(b) is a cross-sectional view of the vibration power generator when its vibrating body is displaced. As illustrated in FIGS. 9(a) and 9(b), an insulation film 902 is provided on a fixed substrate 901. On the insulation film 902, a plurality of first fixed electrodes 903 having a width 2w and a plurality of second fixed electrodes 904 having a width 2w are alternately placed at gaps w/10. On the fixed substrate 901, a hollow spacer 905 is placed. The spacer 905 and the vibrating body 907 are connected to each other via at least two springs 906 in such a manner that the vibrating body 907 is vibratable with respect to the spacer 905. The vibrating body 907 is placed facing the fixed substrate 901, and spaced from the fixed substrate 901 such that electret electrodes 909 formed on the vibrating body 907 are positioned at a distance w from the first fixed electrodes 903 or the second electrodes 904 formed on the fixed substrate 901. On the vibrating body 907, the electret electrodes 909 having a width 2w+w/10 are placed, and the vibrating body 907 and the electret electrodes 909 are sandwiching an insulation film 908. The vibrating body 907, the first fixed electrodes 903 and the second fixed electrodes 904 are arranged such that, when the vibrating body 907 is at a resting position, each overlap between the electret electrodes 909 and the first fixed electrodes 903 or the second fixed electrodes 904 has a width w in the x direction if viewed from a top-down perspective. Further, a lid substrate 910 is provided on the spacer 905 to seal the vibration power generator. The electret electrodes 909 are injected with negative charge, and the vibrating body 907 is vibratabie in the x direction. As illustrated in FIGS. 9(a) and 9(b), the highest positive charge is induced at the first fixed electrodes 903 when change of a first capacitance C1 between the electret electrodes 909 and the first fixed electrodes 903 reaches the maximum value, and the highest positive charge is induced at the second fixed electrodes 904 when change of a second capacitance C2 between the electret electrodes 909 and the second fixed electrodes 904 reaches the maximum value. Such increases and decreases of the electric charge induce a current, and the vibration power generator thus generates electric power (see Tatsuakira Masaki “Power output enhancement of a vibration-driven electret generator for wireless sensor applications”, Journal of Micromechanics and Microengineering, Vol. 21, Issue 10 (October 2011)).
However, because the gaps between the first fixed electrodes 903 and the second fixed electrodes 904 are as narrow as w/10 (a twentieth part of the width of the first fixed electrodes 903 or the second fixed electrodes 904 in the x direction), change of the capacitance C1 between the electret electrodes 909 and the first fixed electrodes 903 as wells as change of the capacitance C2 between the electret electrodes 909 and the second fixed electrodes 904 come close to sinusoidal change. This causes strong electrostatic force between the electret electrodes 909 and the first fixed electrodes 903 or the second fixed electrodes 904 when the vibrating body 907 is around the position of displacement=0, which inhibits displacement of the vibrating body 907 and eventually decreases the power generation. This has been a problem of the above-described conventional generator.
Furthermore, the electrostatic force is weak when the vibrating body 907 is around the position of maximum displacement, which creates another problem that the vibrating body 907 may collide against a stopper and be broken.